1. Field of the Invention
The present invention relates to a diagnostic system of an air-fuel ratio control device for use in an automobile engine.
2. Description of the Related Art
In an engine equipped with an air-fuel ratio control device for maintaining the air-fuel ratio of air-fuel mixture fed into the engine cylinders at the stoichiometric air-fuel ratio, if the air-fuel ratio control device malfunctions, the air-fuel mixture becomes lean or rich. In this case, if the air-fuel mixture becomes considerably lean, the output power of the engine is reduced and the driver becomes aware that a malfunction has occurred. However, if the air-fuel mixture becomes slightly lean or excessively rich due to the malfunction, the driver is not aware of a malfunction and continues to operate the engine. This results in a problem in that a large amount of harmful components such as CO, HC and NO.sub.X will be discharged from the engine. To eliminate this problem, the present applicant has already proposed a diagnostic system which determines whether or not the air-fuel ratio control device has malfunctioned by determining whether the air-fuel fuel mixture should become lean or rich on the basis of a feedback control signal (Japanese Patent Application No. 61-243217).
However, in an engine equipped with a carburetor, if the temperature of the carburetor becomes high, percolation takes place, and as a result, since fuel is forced into the intake passage from the carburetor, the air-fuel mixture becomes excessively rich. Consequently, in this case, if an attempt is made to determine whether or not the air-fuel ratio control device has malfunctioned by using the above-mentioned diagnostic system, although the air-fuel ratio control device has not malfunctioned, the determination result shows that it has malfunctioned. Namely, a problem of incorrect diagnosis may arise.